Semiconductor nanocrystals are tiny crystals typically made of II-VI, III-V, IV-VI, and I-III-VI semiconductor materials that have a diameter between 1 nanometer (nm) and 20 nm. In the strong confinement limit, the physical diameter of the nanocrystal is smaller than the bulk excitation Bohr radius causing quantum confinement effects to predominate. In this regime, the nanocrystal is a 0-dimensional system that has both quantized density and energy of electronic states where the actual energy and energy differences between electronic states are a function of both the nanocrystal composition and physical size. Larger nanocrystals have more closely spaced energy states and smaller nanocrystals have the reverse. Because interaction of light and matter is determined by the density and energy of electronic states, many of the optical and electric properties of nanocrystals can be tuned or altered simply by changing the nanocrystal geometry (i.e. physical size).
Nanocrystals or populations of nanocrystals exhibit unique optical properties that are size tunable. Both the onset of absorption and the photoluminescent wavelength are a function of nanocrystal size and composition. The nanocrystals will absorb all wavelengths shorter than the absorption onset, however, photoluminescence will always occur at the absorption onset. The bandwidth of the photoluminescent spectra is due to both homogeneous and inhomogeneous broadening mechanisms. Homogeneous mechanisms include temperature dependent Doppler broadening and broadening due to the Heisenburg uncertainty principle, while inhomogeneous broadening is due to the size distribution of the nanocrystals. The narrower the size distribution of the nanocrystals, (i.e. a more monodisperse population of nanocrystals) the narrower the full-width half max (FWHM) of the resultant photoluminescent spectra.
A light-emitting diode (LED) is a special type of semiconductor diode. Like a normal diode, it consists of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a p-n junction. As in other diodes, current flows easily from the p-side, or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The wavelength of the light emitted, and therefore its color, depends on the on the bandgap energy of the materials forming the p-n junction. The materials used for an LED typically have a direct bandgap with energies corresponding to near-infrared, visible, or near-ultraviolet light.
Over the past decade there has been significant interest in white light emitting LEDs, liquid crystal display backlighting, projection displays and projectors, outdoor/landscape lighting luminaires, interior illumination in the transportation sector (airplanes, subways, ships, etc.), automobiles, and cell phones and other mobile electronics. Outside of white LEDs there also exists a market for specialty color LEDs particularly aqua, gold, purple, pink, and green used in signage, corporate and product branding and architectural and specialty lighting.
The most common method to achieve white light or specialty-colored light emission from an LED is to combine a powdered phosphor and a blue (450-470 nm emission wavelength) InGaN light emitting diode chip. The phosphor absorbs a portion of the light emitted by the underlying blue LED and down converts the emission of the blue InGaN LED to longer wavelengths. The LED is typically placed and wire bonded in a reflector cup and subsequently coated with a phosphor-containing epoxy. The phosphor is either deposited within the entirety of the reflector cup or is conformal coating on the LED chip itself. The blue light emitted from the LED is absorbed by the powdered phosphor and re-emitted as a light of a longer wavelength, typically yellow. The blue light from the InGaN LED and the generally yellow light from the phosphor combine to form white light. Yttrium aluminum garnet (YAG:Ce3+) is the most common phosphor for this application. A typical emission spectrum of the white light LEDs prepared by combining YAG with a blue light has two distinct peaks.
Since standard white light LEDs use a single broadband yellow phosphor, their respective color temperature is fixed to approximately 6500K. Variation in the underlying LED emission wavelength and the substitution of YAG:Ce for TAG:Ce will shift standard white to 5600K. In any case the color temperatures are fixed. There is significant demand for warm whites having color temperatures correlated color temperatures (CCT) less than 4000 k, particularly less than 3300K and more particularly less than 2800K. There is also some demand for white having CCT greater than 10,000K. None of these white can be reached by the use of single color yellow phosphor plus the underlying blue LED.
“Warm whites” having color temperatures between 3300K and 4500K have been achieved by the addition of “red” Calcium Sulfide and certain Europium doped orthosilicates and nitride based phosphors. However “warm white” LEDs made from a combination of yellow and red have reduced efficiency owing to the lower quantum yield of the red phosphors and also because the red phosphors are broadband emitters. Broadband red emitters have a significant portion of their spectrum in the deep red where the eye is less sensitive. More narrowband green, and particularly red emitters situated at more optimal peak wavelengths would provide increased luminous efficacy warm whites than are presently available.
A second problem associated with traditional white-light LEDs comprising 450-470 nm blue LEDs with a broadband yellow YAG:Ce phosphor is that often the “red”, “green”, or “blue” portions of the emitted spectrum light does not adequately match the spectrum of a true white blackbody spectrum. This leads to a problem that a matter to be displayed in red looks subdued. This problem is often referred to as poor color rendering. Color rendering is an evaluation of how colors appear under a given light source. For example, a shade of red can be rendered more pink, more yellow, lighter or darker depending on the characteristics of the illumination falling on it.
Another method of creating white light with LEDs is by using a multichip array of closely spaced individual “red”, “green”, and “blue” LED chips. If the individual chips are located close to one another the human eye will not be able to resolve individual LEDs and instead blends the individual red, green, and blue emission into white. Because phosphor are not used there is a potential for increased efficiency over white LEDs derived from “yellow” phosphor coated “blue” LEDs. Another advantage is that the intensity of each wavelength component of the multichip array can be varied independently because each chip is separately driven. Thus multichip arrays can be actively tuned to achieve various color whites (different CCTs) or specialty colors. The three LEDs can together emit light having a high color rendering index (CRI), while even higher CRI's can be achieved by adding a fourth amber LED to the multichip array or even more LEDs emitting at different wavelengths.
However, white light emitting multichip arrays suffer from some drawbacks. Green LEDs particularly those that emit light are 555 nm (where the human eye is most sensitive) are notoriously inefficient. Thus, a greater number of green emitting LEDs are needed in the array to achieve sufficient light output. This results in increased costs and reduced efficacy. Furthermore, it is well known that each type of LEDs chips degrade at a different rate over time and have an intensity and peak emission wavelengths that are affected differently by changes in temperatures. Thus, it is a general requirement that multichip arrays incorporate optical detectors and electrical power feedback to each individual chip in order to compensate for different aging and temperature responses.
In addition to white light LEDs, specialty colored LEDs can be produced by the addition of phosphors to underlying blue or UV LEDs. Improved green and yellow-green LEDs may be desirable because, for example, conventional green LED chips are very inefficient, the human eye is most sensitive to green (particularly 555 nm green), and green is used for full color signage or sequential LCD or projection display applications. There is also a need for purple, pink, gold, aqua and other colors that simply cannot be achieved with a single-wavelength LED source.